1. Field of the Invention
The present invention relates generally to a light quantity controlling apparatus for controlling a quantity of light emitted from a light source and, more particularly, to a light quantity controlling apparatus installed in an exposure apparatus and suitable for controlling an exposure quantity by making use of pulse laser beams of an excimer laser, etc. as exposure light, or modulation of energy of far ultraviolet light from a solid-state laser, etc. as exposure light.
2. Related Background Art
There has hitherto been developed an exposure apparatus in which a pulse laser light source serves as a light source for an exposure. A pulse laser beam, however, typically is unstable on the order of .+-.10% per pulse. Besides, the pulse laser beam exhibits such a phenomenon that laser power declines for a short or long period of time. For this reason, such an exposure apparatus has hitherto controlled an exposure quantity by a method involving detecting and adding up a light quantity per pulse beam and continuing an emission of light till a result of this integration comes to a desired value. This kind of conventional exposure quantity controlling method is classified roughly into two methods. One is a modification exposure method of effecting the exposure with pulse beams having a comparatively small light quantity after performing the exposure with pulse beams having a comparatively large light quantity to some extent. The other is a method of making an exposure energy substantially coincident with a predetermined average light quantity value with respect to all the pulse beams needed for one shot of exposure. Every method requires a light quantity regulating element for attenuating the pulse laser beams in terms of light quantity at a predetermined rate. The following are two types of known light quantity regulating elements.
A first example of the conventional light quantity regulating element will be explained. A plurality of attenuating filters having different transmittances are arranged at equiangular intervals to assume a revolver-like shape on a rotary plate. The rotary plate is rotated to make the laser beam incident on any one of the attenuating filters. The laser beam can be thereby discretely attenuated in terms of its intensity.
A second example of the conventional light quantity regulating element will be described. Two plane-parallel plates 3A, 3B have their transmittances which differ depending on an angle of incidence of an incident beam (angular characteristics). The plane-parallel plates 3A, 3B are, as illustrated in FIG. 4, arranged on an optical path of a laser beam LB. The thus arranged two plane-parallel plates 3A, 3B are intended to make an optical path of the laser beam LB when emitted coincident with an optical path when incident. In this case, one plane-parallel plate 3A is rotated clockwise through an angle .theta. with respect to a plane perpendicular to an optical axis of the laser beam LB. The other plane-parallel plate 3B is rotated counterclockwise through the angle .theta.. This angle .theta. is continuously varied, whereby the transmittance relative to the laser beam can be continuously changed.
There exists a case where a desired transmittance is obtained by the method in the first example in the above-mentioned light quantity regulating, per se but can not be necessarily accurately set to a transmittance required when controlling the exposure quantity in the exposure apparatus. Further, it is required that the rotary plate serving as a turret plate be mechanically rotationally driven when changing over the transmittance. This leads to drawbacks in that the changeover takes much time, and a throughput of the exposure step declines.
On the other hand, the transmittance can be continuously regulated by the method in the second example. There is, however, needed a synchronous drive for tilting the two plane-parallel plates in synchronism at the same angles in directions opposite to each other. This leads to a drawback in which the mechanism and the control system become complicated. Additional drawbacks are that response speed is unsatisfactory because of the method of mechanically changing the transmittance, and the throughput of the exposure step decreases.